The invention concerns a process for reducing the benzene content of petrol fractions and isomerising paraffins in the presence of an isomerisation catalyst comprising 4 to 15% of at least one halogen and at least one metal from group VIII deposited on a support composed of a mixture of specific proportions of eta alumina and gamma alumina, in which a feed such as a light reformate and/or a C.sub.5 -C.sub.6 cut is isomerised.
Environmental problems have led to a joint reduction in the lead content and the benzene content in the petrol (gasoline) pool, preferably without reducing the octane number. Catalytic reforming under very severe conditions and isomerisation of normal C.sub.5 -C.sub.6 paraffins with a low octane number are currently the most frequently used processes for producing high octane numbers without adding lead. However, catalytic reforming produces large quantities of benzene with a high octane number. Thus it is necessary to develop new processes which can reduce the benzene content in a petrol while complying with octane number specifications.
The combination of catalytic reforming and isomerisation, consisting in separating the C.sub.5 -C.sub.6 fraction from the reformate, isomerising it and introducing it directly into petrol fractions to improve the octane number, are well known: this is, for example, described in U.S. Pat. Nos. 4,457,832, 4,181,599 and 3,761,392. Isomerisation of the C.sub.5 -C.sub.6 cut from a straight run crude is also well known. The octane number of the cut is considerably improved. The benzene content of the reformate can be reduced in different ways, such as modifying the naphtha cut point between reforming and isomerisation or separating the reformate into two fractions: a heavy fraction (heavy reformate) and a light fraction (light reformate), all the benzene being concentrated in said light fraction. The light fraction is then sent to a hydrogenation unit which transforms the benzene into naphthenes which are then ring opened in an isomerisation unit working under severe conditions. The normal paraffins formed are isomerised using a conventional isomerisation process (U.S. Pat. No. 5,003,118). In addition, European patent application EP-A-0 552 070 concerns a process for reducing the benzene content of a petrol fraction comprising hydrogenation of a feed with the following composition by weight: 40% to 80% of paraffins, 0.5% to 7% of cyclic hydrocarbons and 6% to 45% of aromatics, and with a maximum distillation temperature of between 70.degree. C. and 90.degree. C. The hydrogenation effluent is then isomerised, mixing said charge and/or said effluent with a C.sub.5 -C.sub.6 cut.
European patent application EP-A-0 552 069 further concerns a process for the isomerisation of a feed such as a light reformate and/or a C.sub.5 -C.sub.6 cut in the presence of an isomerisation catalyst which preferably comprises at least one group VIII metal and a mordenite with a Si/Al ratio of between 5 and 50.
U.S. Pat. No. 3,025,248 describes a process for the production of a catalyst with an alpha and beta alumina based support which is treated to convert the aluminas into eta and gamma aluminas. The catalyst also contains a small quantity of platinum or another metal such as palladium, nickel, iron or cobalt, and optionally a trace of chlorine.